A knee is made up, essentially, of four composite parts. The most distal portion is the tibia, which is the larger of the two lower leg bones. The upper surface of the tibia is a generally horizontally oriented plateau. Lining the proximal tibial and distal femoral surfaces are the second important part of the knee, the cartilaginous bearing surfaces, upon which physically rests the distal surfaces of the third portion of the knee, the femoral condyles. Each condyle (the medial, or inward one, and the lateral, or outward one) is generally a toroidal projection on the end of the femur, that can rotate over the bearing surfaces on the tibia. To achieve a wide range of flexion, however, the condyles do not simply rotate on the tibia. They also slide in an anterior and posterior direction, and then revolve generally latero-medially about a centero-medial zone on the tibial plateau. The patella, the fourth part of the knee, in a muscle-tendon mechanism contacts the condyles anteriorly, acting as a pulley for enhancing knee extension. To prevent the femur from sliding off the tibia, the patella is positioned anteriorly of the condyles, between them and connected to the tibia and femur by the patellar tendons and quadriceps muscles respectively.
Prior art prosthetic knee designs have accounted reasonably well for limited rotation and posterior-anterior sliding. For instance, in U.S. Pat. No. 5,282,868 (Bahler) there is described a prosthetic knee that specifically addresses the need for the femoral prosthetic to slide anteriorly while it rotates. The femoral part may, moreover, revolve slightly about its central longitudinal axis. This feature, however, does not enhance flexion. Accordingly, the Bahler knee, while advanced in view of prior designs, does not approach anatomical flexion criteria.
Similarly, in U.S. Pat. No. 5,314,483 (Wehrli et al) a knee prosthesis is described which is capable of sliding and rotation, and which is capable of limited rotation about a centrally located axis. It is similarly limited in flexion.
It is the object of the present invention to provide a prosthetic knee that flexes over a range approximating ordinary knee flexion. In particular, an object of the present invention is to provide a knee capable of flexion in the range of approximately 160.degree., which represents an enhanced flexibility of 25.degree.-45.degree. over currently available prosthetic knees.
A further object of the present invention is to provide a femoral prosthesis that is exceptionally stable after implantation.
In a broad aspect, then, the present invention relates to a knee prosthesis, including: a femoral element; a tibial element; and at least one bearing element between said femoral and tibial component said femoral component having medial and lateral condylar elements that extend posteriorly about 120.degree..